The present invention relates to a method of mass spectrometry wherein reagent ions or electrons are used to transfer charges to analyte ions or analyte molecules so as to cause them to dissociate into daughter ions. The daughter ions can be used to help identify the analyte. The present invention also relates to a mass spectrometer for performing this method.
It is known to use atmospheric pressure electron capture dissociation (AP-ECD) for dissociating ions. This involves reacting all of the ion species generated by an electro-spray ionisation (ESI) ion source with the photo-electrons from a UV lamp. For mixtures of analytes, this can result in complex fragment ion spectra, which include interference from photo-ionised solvent background peaks, dopant ions and their derivatives, un-reacted precursors, as well as mixtures of fragments and charge reduced species from different precursor ions. This complexity can be partially mitigated by using liquid chromatography so as to separate out the components being analysed in time and/or by using subtraction techniques to remove background noise from the spectra. However, assigning precursor ions to their fragment ions from the spectral data can still be challenging. Currently, AP-ECD sources have no means of selecting precursor ions and then associating fragment ions to their precursor ions. This is because in AP-ECD sources the fragmentation occurs upstream of the mass spectrometer and hence before precursor ions can be selected. The above problems limit the analytical utility and commercial acceptance of the AP-ECD technique.
Conventional electron capture dissociation (ECD) and electron transfer dissociation (ETD) have been used in MS/MS instruments so as to associate precursor ions with their fragment ions. Unlike the AP-ECD technique described above, conventional ECD and ETD MS/MS instruments use ion-electron reactions in the ultra low vacuum cell of a Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer or in the low pressure RF containment cell of a quadrupole ion trap or travelling wave ion guide respectively. In these conventional techniques a precursor ion is selected using the MS1 mode of the MS/MS system and is, subsequently subjected to ion-ion or ion-electron reactions. The resulting products include the signature c and z type fragment ions, but for many species an intermediate species is also produced that has not yet dissociated and that is held together by non-covalent interactions. These intermediate products are typically charge reduced precursor ions and are termed ‘ECnoD’ and ‘ETnoD’ ions, rather than ECD or ETD ions, since they have not dissociated. Fragmentation of the non-dissociated intermediate species can be assisted by additional ion activation so as to further improve the abundance of ECD and ETD c and z fragment ions.
It is desired to provide an improved method of mass spectrometry and an improved mass spectrometer.